Lighting systems utilizing multiple emitters or light emitting diodes (hereinafter “LEDs”) are used in a variety of applications including but not limited to retail displays, refrigeration and freezer door systems, under cabinet lighting, track lighting, and cove lighting. Continuous strings of LEDs are often used in these applications and may be individually wired together or soldered onto printed circuit board substrates. Typical applications use standard circuit board materials such as Flame Retardant 4 (hereinafter “FR4”) or Metal Core Printed Circuit Boards (hereinafter “MCPCBs”), which are typically rigid.
Designers of high power LED systems quickly adopted these standard circuit board materials due to their low cost and wide spread availability. The broadest applications focus on the use of aluminum clad MCPCBs in an attempt to transfer the high amount of heat generated from high power LEDs (1 W or higher). FR4 materials are known to be poor thermal solutions for high heat circuits as the electrically insulating layer results in poor thermal conductivity from the LED heat sink slug to the assembly heat sink. Thus, LED manufacturers typically recommend MCPCBs in place of FR4 for use in high power LED applications, such as Philips Lumileds Lighting Company in its Custom LUXEON® Design Guide, Application Brief AB12 (3/06).
Standard circuit board materials are typically rigid material in form and do not conform to irregularities in heat sink surfaces. The standard circuit board materials are commonly screwed onto metal heat sinks with thermal grease placed between the aluminum clad and the assembly heat sink with some applications using thermal tape in place of mechanical fastening and thermal grease. Thermal grease and adhesive thermal tapes fill small voids of 0.002 inch or less due to surface irregularities but are not sufficient to fill larger voids or air gaps commonly occurring between standard circuit board materials and the assembly heat sinks due to the heat sinks being made out of flat, twisted, or curved surfaces. Issues with poor conductive interfaces quickly lead to failed LED systems in applications in the field due to voids or air gaps between the MCPCB or other substrate and the intended heat sink. These applications typically suffer from poor transfer of heat from the LED source to the heat sink due to poor conductive surface contact due to the board being too rigid to conform to the heat sink shape or to poor heat transfer through the board due to MCPCB thickness and layered structure properties.
More exotic solutions are available for LED systems including metal clad boards with thin, higher thermal conduction insulating layers or printed ceramic circuits onto steel or aluminum substrates. These materials provide better thermal management of high power LEDs over standard circuit board materials due to their higher thermally conductive materials but suffer from the same heat sink interface issues as the standard circuit board material and further at a severe sacrifice to system costs. Printed circuit board materials, such as T-lam™ Thermally Conductive Printed Circuit Board Materials by Laird Technologies, are one example of the more exotic, costly materials used to increase the thermal conductivity through the electrical insulating part of the structure.
LED systems are sought as energy efficient solutions to replace many established, less efficient lighting systems using common lighting sources such as incandescent, halogen, and fluorescent lighting sources. The introduction of LED systems into many general illumination and display applications is limited due to the high front end costs of LED systems. While the performance of the LEDs continue to climb and the cost of the LEDs continues to drop at rates consistent with other semiconductors following Mohr's Law, little is being done to reduce the costs related to the support systems necessary for high power LED systems such as board substrates.
The costs of assembling LED systems into lighting fixtures are further increased due to the labor intensive nature of applying rigid board materials with thermal grease, paste, or adhesive along with mechanical fastening of the rigid board materials to heat sinks. Flexible tape and reel solutions have been developed for low wattage LED systems, however, these solutions are not applicable to high wattage LED systems due to the amount of heat generated by the higher wattage systems and the inherent poor thermal properties of the flexible materials such as DuPont™ Kapton™ polyimide film.
Therefore, there exists a need for a cost effective, high thermal performance substrate or solution for use with high power LED systems. The present invention addresses the problems associated with the prior art devices.